Core body temperature is the temperature of the vital organs of an individual. An abnormally elevated body temperature occurs when an individual is in a febrile state and can result in denaturation which is a process that causes irreversible loss of protein function, ultimately leading to cell death. An abnormally low body temperature causes an individual to be in a hypothermic state which can affect and impair the rate at which chemical reactions in the body take place and possible lead to respiratory or circulatory failure. For many years, the standard for normal or baseline body temperature has been 98.6° F., or 37° C., being the temperature at which the body is attempting to stabilize. However, research has proven that normal body temperature is actually a range of temperatures. According to the American Medical Association, normal body temperature of an individual can range from approximately 97.8° F., or 36.5° C., to 99° F., or 37.2° C. Typically, the body maintains a normal or baseline temperature generally within the narrow range of 36.5-37.5° C. Skin temperature is generally recognized as being 2-3° C. cooler than core, the actual gradient being dependent on many factors, including the ambient temperature of the environment surrounding the body and vasomotor tone. The specific normal or baseline measured temperature of an individual depends on a variety of factors. For example, time of day, recent activity, fluid and food consumption, measurement location and/or measurement technique can affect the detected body temperature of an individual. Also, normal body temperature of a group of individuals having similar demographics may vary based on these or other factors including age, metabolic rate, gender and if a disease condition is present.
Through monitoring of an individual's body temperature over time, the actual normal body temperature or range of temperatures of a specific individual can be determined. Knowing this vital statistic is important for preventing the occurrence of temperature extremes which can cause significant damage to tissues and cells of the human body. Additionally, an elevated body temperature can result in a febrile seizure, which is a brief convulsion that occurs repeatedly in association with a fever in infants and children particularly. Febrile seizures are associated with a rapid onset fever and occur in children between the ages of 6 months and 6 years of age. Although a febrile seizure does not typically result in long-term or permanent damage to the individual, there is an associated risk of bodily injury, as with any type of seizure.
True core body temperature is the temperature of the arterial blood flow from the heart and is most accurately measured at the center of the heart. Measurement at this particular location would require pulmonary artery catheterization, which is not appropriate under most circumstances due to the invasive nature of such a procedure. Consequently, body temperature measurement that provides a result closest to the blood temperature of the individual must be measured at a convenient location that is closest to core body temperature. The most widely accepted locations for measurement of body temperature are either external or externally accessible to the body or do not pose significant risk of injury to the individual. Typically, these locations include oral, axillary, rectal, and tympanic. However, the temperature measurement at any of these sites is not true core body temperature and therefore has an associated error or variance from that core body temperature, depending on the location.
One factor affecting the accuracy of temperature measurements is that different measurement locations have different rates of perfusion. Perfusion generally refers to the release of nutrient compounds needed by the cells to perform vital functions. Perfusion is further defined as the amount of arterial blood flow required to accomplish the release and distribution of nutrient compounds to the different areas of the body. Accordingly, perfusion can be correlated to factors indicative of blood flow such as blood temperature, because an area that is properly perfused has an adequate blood supply flowing through that area.
The hypothalamus of the human body attempts to maintain the body in a state of homeostasis, which is a metabolic equilibrium of the bodily functions. However, when this metabolic equilibrium is affected by ambient temperature, a hypothalamus set-point for body temperature related reactions may be triggered resulting in decreased blood flow to areas of the body. As blood flow travels farther from the heart and other vital organs, the effect of ambient temperature on the particular area of the body away from the heart is increased. For example, when the ambient temperature is lower than normal, the body will decrease peripheral blood flow to the extremities in order to maintain the homeostasis and associated core body temperature of the vital organs. The decreased peripheral blood flow is directly correlated to decreased perfusion, which leads to a lower skin temperature.
Blood supplies traveling through different areas of the body have different rates of temperature change corresponding to rising and falling body temperature. The amount of time for fluctuations in temperature to be reflected in the blood supply is largely varied among the detection locations on the body. The error or variance is also affected in large part by environmental conditions. Further, each site has error variables unique to that site that influence the measurement result.
Oral temperature is a convenient non-invasive measurement location and is an accepted equivalent for core body temperature, especially in clinical settings. The tongue has a relatively large blood flow with a temperature that mirrors that of core body temperature. However, the activity of an individual, including coughing, drinking, eating, and talking, can lower the detected temperature of the individual and produce an erroneous result. Although widely used, this method of temperature measurement depends upon proper position of the measuring device and cooperation of the patient. Recommended measurement time is three minutes to get an accurate reading.
Axillary temperature is another convenient and non-invasive site for measuring temperature. Axillary temperature can be taken externally in the armpit between two folds of skin of the armpit and arm. The accuracy of this measurement is typically dependent upon the measurement being taken relative at a location proximate to the artery on the body side. The axillary site can be adversely affected by ambient temperature in that an exceptionally cool or warm environment will produce an erroneous result. Further, the shape of the armpit affects the result because a hollow armpit is less insulated and provides increased exposure to ambient temperature of the environment. Temperatures taken in this manner tend to be 0.3 to 0.4° C. lower than corresponding temperatures taken orally. The measurement time is similar to the oral temperature technique or longer.
Rectal temperature is measured internally in the rectum. It is the least time consuming, with a typical measurement time of one minute. This is particularly important when measuring the temperature of infants, as they tend to move around, which causes additional error in the measurement. It is, however, the most uncomfortable location for measurement. The increased accuracy over oral and axillary measurements stems from the fact that the rectum is well insulated from the environment and the resulting temperature measurement is a closer match to an individual's core temperature than the temperatures measured at either the oral or axillary sites. Temperatures taken rectally tend to be 0.5 to 0.7° C. higher than corresponding temperature readings taken by mouth.
Although rectal temperature measurements are more accurate, the measurement process has associated disadvantages. This particular method poses a risk of injury to the individual because the insertion of the temperature probe into the rectum may cause perforation of the delicate tissues, in addition to the risk of infections and other illnesses stemming from lack of hygiene relating to the measurement device and/or its use. Also, rectal temperature responds more slowly than oral temperatures to changes in heat input and loss because any matter contained within the rectum acts as insulation and any rapid body temperature changes are not immediately reflected.
There are two locations in the ear which are also appropriate for temperature measurement. The first location is the external portion of the ear canal. The ear canal is a convenient, non-invasive location but is subject to significant influence by environmental conditions and the cooling effect of these conditions on the body. The second location is the tympanic membrane which is located deep inside the skull and is not subject to the same influences as the ear canal. Tympanic temperature has also become a common measurement technique in recent years. Tympanic temperature is a close reflection of core body temperature because the eardrum shares the blood supply with the hypothalamus which controls temperature. Temperature changes are reflected sooner and are more accurate. To measure the temperature at the tympanic membrane, however, a long thin thermocouple probe has to be inserted into the ear causing a great deal of discomfort to the individual. The thermocouple probe must contact or at least remain close to the very delicate tympanic membrane which entails a cooperation of the individual and a risk of injury.
A wide variety of devices and techniques are know for the measurement of body temperature, most of which are directed to static, as opposed to continuous, measurements. The most accurate devices and methodologies for temperature measurement are, unfortunately, the most invasive and include pulmonary artery/thermal dilution catheters, esophageal temperature probes and indwelling bladder and rectal temperature probes. Pulmonary artery/thermal dilution catheters are the most accurate method of temperature measurement because of the ability to continuously monitor the temperature of the pulmonary outflow of the heart. However, because these methods are invasive and impractical, other devices have been developed to more conveniently measure the temperature of an individual, even on a static basis.
The glass mercury or expanding liquid thermometer has been used to measure temperature for many years, however the accuracy of this device is questionable, in part because its accuracy significantly depends on the time at which it is properly located and the reader properly interpreting the scale. This accuracy deficiency is partially due to the limited number of locations for measurement while using the device, which include oral, axillary and rectal. Studies have revealed that glass mercury thermometers demonstrate errors on the order of 0.5° C. or 0.9° F. at normal body temperature and errors of greater magnitude when an individual is febrile. In addition, accidental breakage and disposal is cause for concern when using a glass mercury thermometer. When liquid mercury is spilled, it forms droplets that emit vapors into the air which are odorless, colorless and toxic. Because mercury is poisonous and hard to clean up if spilled, these thermometers are less common today and have actually been banned in some locations. Also, there is no ability of the device to obtain and record a history of the temperatures of an individual because only individual serial measurements are recorded on this simple measuring device. Continued long-term temperature measurement which is not continuous can be troublesome to the ill individual who must be awake for each measurement. The electronic thermometer, also called the digital thermometer, is considered more accurate than a glass mercury thermometer, but essentially provides similar functionality with a small improvement in convenience.
The chemical thermometer, designed to be a one-time use or disposable product, is a type of probe thermometer. An example of this type of thermometer is the Vicks Disposable Thermometer, Model V920. This device is a paper device with heat activated chemical dots superimposed on the surface. The dots change color based on the temperature measurement. This device provides some advantage in that it can be thrown away after its use so that germs and bacteria do not contaminate the device for continued use. However, this particular type of thermometer strip has been found to be imprecise, inaccurate, inconsistent and yields frequent false-positive results.
Many of the recent developments in the field of temperature measurement are directed toward improving comfort and convenience for the user, such as the use of a curved, rubber accessory or probe that is conformed and flexible to fit over the teeth and inside the mouth to rest more easily on the jaw to garner greater application consistency. These efforts can also be counterproductive. In one example, a pacifier-like probe is utilized to allow an infant to be monitored with a familiarly shaped device. The natural and reflexive sucking action of the infant, however, causes the signal from this device to be noisy and inaccurate. These improvements have therefore been directed toward ease of use issue but little has been accomplished in terms of increasing accuracy and consistency completely apart from technique and user error. Additionally, all of the preceding devices are directed toward static measurements. In most, if not all circumstances, these devices are entirely impractical as continuous temperature monitors for ergonomic, safety, convenience and data retention reasons.
Other newer techniques and devices include sensing diaper urine or bowel movements in a diaper, immediately after release from the body when the substance is at core temperature. The limitation is that this is entirely event driven and must be properly anticipated, in the proper location, and must be able to detect the peak temperature to record the measurement before cooling or heating up. Additional practical considerations include the need to dispose of or clean the product because the sensor/device is now soiled.
An infrared thermometer is a non-contact temperature measurement device that detects infrared energy emitted from an individual and converts the energy to an electrical signal that can be displayed as a measurement of temperature after being corrected for variation due to ambient temperature. An infrared thermometer can be used at a variety of locations and provide significant advantages. Infrared thermometers can be used to take temporal membrane measurements which have more recently been reported to have strong correlation to pulmonary arterial temperature, but have also become popular especially in infant monitoring because they don't require the measurer to disturb the infant through an orifice or under the arm, especially if frequent readings are required or prescribed to be performed. The main disadvantage of an infrared thermometer is that the device is highly dependant on the operator's technique. It can be difficult to get a consistently accurate reading without a consistent method of use. Also, the cleanliness of the infrared lens can significantly impact the results of measurement. Further, infrared thermometers typically do not account for the effects of ambient temperature on the skin temperature measurement of the individual.
In most cases, there is also the traditional trade off between cost and accuracy. This is exacerbated in this field, especially within the realm of disposable products. Disposable products are increasingly popular in light of concerns regarding hygiene. This is most applicable to institutional applications. Disposability, however, necessitates a firm cost ceiling for any product, which in turn limits the ability of the device to provide more than the most limited functionality.
In many situations, temperature readings, together with the data, diagnoses and other information extrapolated or derived from the temperature readings, would be more useful and accurate if made continuously rather than the periodic, static measurements now commonly made and described above. Several devices and techniques have been proposed to facilitate continuous measurement.
Exterior skin has traditionally not been considered an appropriate location for temperature measurement, even when measurement is taken near a surface artery. This is, in part, because skin temperature measurements suffer from significant noise from peripheral shutdown, skin insulation, activity and environmental and internal (hydration) convolutions. Even so, skin locations are much less invasive and potentially comfortable for continuous wear of a temperature monitor. These monitors can also be protected from environmental noises by clothing, diapers, attachable bands and the like.
A Wireless Thermometer manufactured in Taiwan and Japan by Funai and marketed by Granford Marketing and Management Services under a variety of trade names provides a transceiver device which is clipped onto clothing or diaper of the patient to be monitored. A sensor is mounted internal to the clip and is intended for direct contact with the skin. The device relies upon the article of clothing or diaper to maintain the contact between the skin and the sensor. The sensor records the temperature and displays the reading on an LCD screen. The transceiver device is paired to a receiver unit by wireless transmission which receives the temperature data and may be preset to sound an alarm if a certain temperature threshold is reached. No provision is made for storage of any historical data. A number of other prior art devices do provide this functionality.
Rubinstein, U.S. Pat. No. 6,852,085, issued Feb. 8, 2005, for a Fever Alarm System, discloses a continuous body temperature measurement device. The device comprises a microprocessor having two thermistors that continuously measure skin temperature and ambient room temperature for calculation of body temperature. One thermistor lies adjacent to the skin and is insulated from the surrounding environment. The second thermistor is exposed to the ambient room air and is not in contact with the skin. The device measures both skin and ambient room temperature and then transmits the calculated result through an RF transmitter to a display unit which displays the current temperature of the individual. The device further includes an adjustable alarm that is triggered when a certain predetermined temperature threshold is reached.
The device continuously measures both skin temperature and ambient temperature, and must first log a history of ambient room temperature for thirty minutes before a first result is calculated. The thirty minute delay in accounting for the ambient room temperature can be life-threatening when monitoring a febrile individual. The output of the device is a calculation, which is not based on the actual measurement history of the individual's detected temperature or on a correlation to that specific individual's physiology, physiological performance, activity and core temperature. Instead, the device obtains this information from programmable read-only memory containing tabular data of analytic values. The tabular data is derived by a process of data to data mapping in which a particular output is generated for a particular set of possible inputs. The data contained in these look-up tables is taken from previously determined experimental data of body temperature versus skin and ambient temperature and the relationship and effect on each other over time. The data requires an initial storage of reference values and has no relationship to the input for a specific individual.
Pompeii, United States Patent Publication No. 2003/0169800, for an Ambient and Perfusion Normalized Temperature Detector, published Sep. 11, 2003, discloses an infrared thermometer that estimates core body temperature by measuring the axillary and/or tympanic temperature of adults with an infrared sensor. The device calculates core body temperature using the arterial heat balance equation which is based on heat flow through thermal resistance from an arterial core temperature to a location of temperature measurement to the ambient temperature. The arterial core temperature is calculated based on ambient temperature and sensed skin temperature. Pompeii suffers from the deficiencies described above with respect to infrared thermometers, generally, including technique and lens quality. In addition, Pompeii's calculation does not use a direct measurement of ambient temperature. Ambient temperature is an important factor in determining skin surface temperature because the effects of ambient temperature on the skin can grossly affect the resulting measured skin temperature. To account for ambient temperature, Pompeii calculates the core temperature of the individual using the sensed temperature of the detector as the ambient temperature, with 80° F. being the presumed value for the detector. However, the detector may be either cooler or warmer than the surrounding ambient environment, affecting the accuracy of the result of the calculation. The accuracy of the final temperature calculation may be improved through adding or subtracting 20% of the difference between 80° F. and the actual temperature of the device.
Specifically, in other methods of axillary thermometry, the difference between skin temperature and ambient temperature is calculated as being a weighted coefficient determined by approximating h/pc where h is an empirically determined coefficient which includes a radiation view factor between the skin tissue and the ambient temperature, p is the perfusion rate and c is blood specific heat. The approximation of h/pc under normal circumstances for afebrile individuals varies over a range of at least 0.09 to 0.13 corresponding to a variation of about 30%.
Instead of assuming that the ambient temperature, estimated by Pompeii to average approximately 80° F., is always the same as the detector temperature, Pompeii weights the sensor temperature by 20% as the sensor temperature varies from 80° F. For example, if the detector is sensed to be at 80° F., the corresponding ambient temperature used in the calculation is not corrected because the detector temperature and the ambient temperature are assumed to be equal. However, as the temperature of the sensor increases or decreases from 80° F., the ambient temperature used in the calculation of body temperature is varied by 20% accordingly in the same direction.
Fraden, United States Publication No. US 2005/0043631, for a Medical Body Core Thermometer, published Feb. 24, 2005, discloses a device intended primarily for surface temperature measurements. The device calculates core temperature by sensing the temperature of the skin while accounting for the sensor temperature and ambient temperature. The device has a first sensor for measuring skin temperature as a function of the thermal resistance of the user. The device has a second sensor which measures a reference temperature of the measuring device. Although Fraden accounts for ambient temperature, the device is not adapted to measure ambient temperature which is an important factor in calculating an accurate measurement of skin surface temperature. Fraden attempts to eliminate ambient temperature from the calculation by using a pre-warming technique comprising an embedded heater to heat the device to a temperature that is near the potential skin temperature.
Fraden further utilizes an equation that requires multiple measurements of skin temperature to account for the effects of ambient temperature. The equation does not require a detected ambient temperature, nor does Fraden measure the ambient temperature. The Fraden device does require at least three temperature measurements to determine skin temperature. The first measurement is the detected temperature of the device before it is placed in contact with the skin. The second measurement is an initial skin temperature measurement detected upon the placement of the probe on the skin of the user. The third measurement is the detected temperature corresponding to an altered temperature after the device is placed in contact with the skin. This altered temperature measurement is related to the increased skin perfusion resulting from the surface pressure exerted on the skin by the device. Specifically, when surface pressure is exerted on the skin of an individual, the perfusion of the stressed skin is increased due to the vasodilatation of the blood vessels at that particular site. This results in an increased blood flow at the site and possibly a more accurate skin temperature measurement.
Based on the multiple measurements taken with the Fraden device, the skin temperature of the individual is calculated. Core body temperature is calculated using experimentally determined constants and the calculated skin temperature. Although the blood flow to the area is increased so that skin temperature can be more accurately measured, ambient temperature still has an effect on the skin temperature, and the result of the calculation is in conflict with the true core body temperature of the individual.
Matsumura, U.S. Pat. No. 5,050,612, for a Device for Computer-Assisted Monitoring of the Body, issued Sep. 24, 1991, discloses a method for estimating core body temperature at the skin surface comprising monitoring the skin surface temperature at a location on the body. Matsumura discloses that ambient temperature affects the temperature measured at the skin surface, but a first device contemplated by Matsumura uses only a skin temperature sensor and insulation to prevent the ambient temperature from affecting the skin temperature measurement. Insulation of at least a four square centimeter area is used in connection with a temperature sensing means to insulate the skin from the surrounding environment such that the skin could theoretically adjust more closely to core body temperature. Matsumura further discloses a second device that includes a second sensor for measuring the temperature of the ambient environment and in addition to lesser quantities of insulating material to insulate the skin from the ambient environment. However, the insulating material is required in a lesser quantity.
Data is detected by both the first and second sensors and used to manually calculate the core body temperature of the individual. The user creates a look-up table by charting a record of the skin temperature and corresponding ambient temperature. Matsumura states that by correlating skin temperature as it exists at a particular ambient temperature, core temperature can be determined. Matsumura does not disclose how core body temperature is determined but allows for the use of a table to correlate measured and calculated temperature. The determination of ambient temperature can also be affected by the amount of insulation used in constructing the device. For the first device, Matsumura requires a minimum of four square centimeters of insulation to be placed around the sensor to shield it from the environment. For the second device that is equipped with an ambient sensor, Matsumura is not specific but only states that that the required insulation is less than what is required for the first device. If wear of the device is not consistent in that the insulation is removed and changed during the charting of reference temperatures, the effect of the ambient temperature may not be a consistent result with respect to skin temperature. The insulation shields the skin sensor from the environment and a certain temperature is detected based upon the amount of insulation used. If the amount of insulation varies between the placement of the sensor device on the body, the accuracy of the user created chart is affected.
Ward, U.S. Pat. No. 4,509,531, issued Apr. 9, 1985 for a Personal Physiological Monitor, discloses a continuous physiological monitor that detects changes in either galvanic skin resistance, temperature or both in order to detect the onset of hypoglycemic states in a diabetic individual. A temperature reference is automatically established by the device as it is worn by the user. The skin temperature of the user is monitored by a skin temperature sensor, and once the measured temperature drops below the temperature reference, an alarm sounds. Ward mentions that ambient temperature affects the skin temperature measurements of an individual but does not provide a means to measure or a method to account for ambient temperature.
Dogre Cuevas, U.S. Pat. No. 5,938,619 for an Infant External Temperature Monitoring Transmitter Apparatus with Remotely Positionable Receiver Alarm Mechanism, issued Aug. 17, 1999, also discloses a device to detect changes in skin temperature. However, although the device comprises a skin temperature sensor, it does not provide a mechanism to measure ambient temperature. Further, Dogre Cuevas does not contemplate ambient temperature as having an effect on skin temperature.
Continuously measuring body temperature of an individual can be beneficial in monitoring the well-being of that individual and provides a better indication of the individual's normal body temperature. Having knowledge of the normal body temperature of an individual may aid in the prevention of life-threatening conditions can be prevented or detected quickly. Temperature measurement devices exist that provide both serial and continuous temperature detection and measurement of the user. However, the serial temperature measuring devices are not very helpful in monitoring the normal body temperature of an individual for quick identification of an abnormal temperature unless monitoring is done manually by the user or caregiver. Further, the current temperature measurement devices that provide continuous measurement provide less than accurate results because the devices fail to account for conditions that affect skin temperature, including activity, personal physiology and diaper conditions for both infants and adults.
Additionally, many prior art devices base the calculations of core temperature upon certain measured alternative conditions, such as skin temperature and utilize standardized conversions or tables of data to correlate these readings to a meaningful output temperature.
Therefore, what is lacking in the art is a continuous temperature measurement monitoring device that promotes long term wear and provides an accurate measurement of the actual core body temperature of an individual. Additionally, what is lacking is a multisensor device which may utilize additional environmental and physiological parameters to increase the accuracy of the temperature output. These temperature measurements may also be utilized to provide activity and conditional information about the individual which may be useful for informational, diagnostic and other purposes.